1. Field of the Invention
The present invention relates to an improved data processing system and more particularly to a method and apparatus for reporting and handling error conditions in a printer. Still more specifically, the present invention relates generally to a method, data processing system, and computer usable program code for sensing paper movement, out-of-paper, and cover open in a printer.
2. Description of the Related Art
Printers are currently found in many forms, however all printers share common characteristics, such as a print head, a platen, and a control mechanism. The control mechanism controls the motion of the print head relative to the paper, selects a character to be printed, and advances and retracts the paper as necessary.
It is undesirable for a printer to operate without paper. Ink-based printers that are operated without paper will transfer the ink into the platen which may in turn stain the back sides of subsequent sheets of paper and possibly damage print writes in the print head. Thermal printers operated without paper may overheat because paper is used to absorb the heat generated by the print head during printing operations or cause excessive wear to the print head because it is running on the platen rather than the paper. Also, any printer that operates without paper will cause frustration when documents must be reprinted. Most modern printers, therefore, include a control mechanism to stop the printer if paper runs out and to prevent the printer from starting to print unless an adequate paper supply is present. This control mechanism typically includes a limit or proximity switch to detect the presence or absence of paper.
Most modern printers are also enclosed in covers or cases with hinged or removable sections that open for access to the printing and mechanical areas. These printers are not designed to be operated with the cover open, as the internal mechanism may be at high temperature or include numerous moving parts. This situation is especially true in the case of a thermal printer where the print head can be damaged if the print head is driven when the cover is open and the print head is not against the platen and nothing is present to absorb heat from the print head. Therefore, these printers typically include an interlock that will prevent the printer from operating with the cover open. The usual interlock includes a limit or proximity switch used to detect whether the printer is open or closed.
The control switches used to detect paper and determine whether the printer is open must be durable and capable of handling a large number of cycles without failure, as failure of these switches can result in damage to the printer or injury to the operator. As a result, the switches used for these tasks are usually expensive. The use of separate switches for these tasks adds substantial expense to the cost of the printer because of associated hardware costs.
If the paper is not moving when the printer paper feed rollers are driving, either a paper jam is emanate or the paper feed system has failed. In either case, it is important that the system is alerted. Printers can be damaged by severe paper jams and time can be wasted while an operator has to clear the jam and reprint the document. This situation is especially important in point-of-sale printers. For example, IBM's Model 3 point-of-sale printer uses a roller driving an emitter wheel to monitor paper motion to ensure reliable journal printing.
Separate emitter wheel mechanisms to track paper movement, cover open sensors, and paper out sensors complicate the printer design, increase total printer cost and can reduce the reliability of the printer.